linux-kernel - RE: [PATCH] hex2bin: make the function hex_to

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Message-ID: <alpine.LRH.2.02.2204250701410.10912@file01.intranet.prod.int.rdu2.redhat.com>
Date:   Mon, 25 Apr 2022 07:04:05 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
cc:     "'Linus Torvalds'" <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andy@...nel.org>,
        device-mapper development <dm-devel@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Mike Snitzer <msnitzer@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Milan Broz <gmazyland@...il.com>
Subject: RE: [PATCH] hex2bin: make the function hex_to_bin constant-time



On Mon, 25 Apr 2022, David Laight wrote:

> From: Linus Torvalds
> > Sent: 24 April 2022 22:42
> > 
> > On Sun, Apr 24, 2022 at 2:37 PM Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:
> > >
> > > Finally, for the same reason - please don't use ">> 8".  Because I do
> > > not believe that bit 8 is well-defined in your arithmetic. The *sign*
> > > bit will be, but I'm not convinced bit 8 is.
> > 
> > Hmm.. I think it's ok. It can indeed overflow in 'char' and change the
> > sign in bit #7, but I suspect bit #8 is always fine.
> > 
> > Still, If you want to just extend the sign bit, ">> 31" _is_ the
> > obvious thing to use (yeah, yeah, properly "sizeof(int)*8-1" or
> > whatever, you get my drift).
> 
> Except that right shifts of signed values are UB.
> In particular it has always been valid to do an unsigned
> shift right on a 2's compliment negative number.
> 
> 	David

Yes. All the standard versions (C89, C99, C11, C2X) say that right shift 
of a negative value is implementation-defined.

So, we should cast it to "unsigned" before shifting it.

Mikulas